The present invention relates to a method and apparatus according to the preambles of the independent claims presented later in this patent application for treating organic waste, especially for decomposing waste biologically and/or chemically. The invention also relates to the use of the apparatus.
Waste containing organic matter derived from livestock husbandry, as do many types of community waste, contain a great deal of useful soil-improvement material. All waste is not, however, suitable to be used as such for soil-improvement because of its odour etc. Moreover, most of the waste contains a lot of material that is unsuitable for soil improvement because of its shape or composition. Besides, since a lot of waste is produced and accumulated in places where its transport for soil improvement over long distances would be difficult and expensive; its exploitation is not optimal today.
It is a known practice to treat waste containing organic matter biologically by decomposition, in which process it is also possible to eliminate problems caused, for example, by odour. Waste can be decomposed in bioreactors by bacteria-mediated anaerobic digestion and/or by aerobic digestion using composting. The biogas formed in decomposition can be used, for example, in energy production. The solid waste formed in decomposition and energy production, which is odourless and contains large amounts of nitrogen, phosphorus and potassium, is easier to process further for soil-improvement material.
In digestion, the waste and a specified amount of liquid are introduced into a closed decomposition reactor, where anaerobic bacteria are allowed to act so as to decompose the waste sludge formed in the reactor. Among other substances, methane-containing biogas is formed in this reaction, which is recovered for re-use, e.g. for combustion. Digestion is an advantageous way to treat various types of biological waste, such as household and agricultural waste, including waste from the food industry. Also sewage sludge from waste water plants can be digested. After digestion and collection of biogas, the sludge is in a more advantageous form for all kinds of further treatment. Liquid separated from the treated sludge can be recycled in the decomposition process, or purified. Dry matter separated from the treated sludge can be composted to form nutrient-rich humus, which can be further used in soil improvement.
It is known that decomposition reactions are relatively time consuming. In addition, decomposition has often been shown to be incomplete and unsatisfactory. Part of the sludge tends to emerge from the reactor as such, or it is only partly decomposed. It is shown in WO publication 95/32158 that one reason for this is uneven moisture content of the waste material. The proposal for improvement presented in the WO publication is to feed waste or sludge into the bottom part of a decomposition reactor having a special shape, and then feed liquid into its upper part. In the reactor, the waste is arranged so as to travel from bottom to top. The biogas formed from the waste in the decomposition reactor is dissolved into the liquid, from where the biogas is collected in a separate bioreactor. The aim is thus to achieve a better result by selecting a special reactor type and special waste sludge treatment process.
Earlier it has been proposed in the European patent application EP 0 679 719 A2 that solid waste should first be separated from mixed waste containing organic waste such as household waste using a screen or metal separator, after which the waste should be ground into a pumpable sludge, which in turn can be treated by hydrolysis and digested using anaerobic digestion. The aim is to achieve complete sludge digestion using two or more bioreactors arranged in series.